For four years we have investigated the structure, function and synthesis of a set of tRNAs synthesized by bacteriophage T4. When we began this work it was already clear that there are two different sorts of changes in the tRNA complement of the T4-infected cell. At least one tRNA of the host, a tRNA Leu, is inactivated by an endonucleolytic cleavage (1-3). Simultaneously seven new tRNA species coded by T4 appear in the infected cell (4-6). These species have distinct chromatographic properties (7,8), can be labeled with radioactive (35)SO4 ion (4-9) following infection, and specifically hybridize to T4 DNA (5,6,9,10). We hoped that the T4 tRNA would provide a tool for studying tRNA structure-function relationships and for investigation of the role of tRNA in controlling the metabolic state of the cell. Our studies to date have convinced us that the T4 tRNA system is indeed a useful one. In addition to providing answers to the questions we had asked, it is now apparent that our study of the synthesis of these tRNAs will lead us into the study of RNA maturation, since this system provides substrates for the purification and characterization of enzymes which are involved in post-transcriptional cleavage of RNA. The T4 tRNAs are not obligatory for growth in the normal laboratory strains of E. coli. Thus they have dispensable functions and as in the case of the rII gene the tRNA genes can be easily manipulated genetically. These genetic studies have greatly enhanced the value and importance of the T4 tRNA system. We now have available a large collection of mutants that can be used to probe many aspects of tRNA structure, function and synthesis.